Tracking Issue????

[swampy2646]

Hello to all, I have a 1991 2500 SCR with a 350 Mag. MPI. With a Bravo 1 drive. Im swinging a 4 blade 16p stainless. I always have had a problem with tracking from starbord to port at low rpm. I guess im kinda used to it, but my wife and kids hate to pilot the boat because of it. Anything to do to correct this?

[GlennW]

Alot of boats do this, you might try trim tabs. I put one of those fins on a couple of my smaller boats and it worked but they were alpha drives.

[swampy2646]

yeah, but thats more for a planeing or porpoising issue.

[mcoffey]

That is caused by Vortex Shedding and is very common.
Check out this write up about it.
http://www.iboats.com/review_monthly..._article2.html

[seapuppy]

Mcoffey...
unfortunately what he's talking about is bow wander..it's a simple physics problem where the bow is trying to be pushed off to the side by the drive in the back...vortex shedding occurs when a cylinder is shedding the current eddy and causes a vortex ....boats do not experience VS but do experience some form of bow wander.....
http://en.wikipedia.org/wiki/Vortex_shedding

in a single drive boat...especially an outboard or i/o..put the drive all the way down...tabs down...and make small corrections just as the boat starts to make a direction change.....bow wander is caused the drive trying to push the bow(which is lighter than the stern ) out of the way....it happens to almost every single engine boat to some degree..

SP

[pascavone]

hold the wheel when the engine is off and have someone wiggle the outdrive, in trailer mode up.

I'd say after 20 years your main "tiller" pin and bushing are shot.

this will require the whole transon assemble to be remove, to get at the bushing.

they make a kit to cut a hole transom from the outside of the boat, but people have mixed feeling on it.

[mcoffey]

SP, I am glad to hear you are sticking around. It would have not been the same around here without you.
But on this subject, I will have to agree to disagree with you here. I would like to think that this guy knew what he was talking about (though maybe I put my trust in the wrong people?).
I will quote:
"Roy Ellis, senior project manager of product development for Monterey Boats, said it’s the result of water rushing aft along the sides of the boat and then collapsing into the hole behind it.
This creates swirling vortices of water that exert suction on each side of the transom, pulling the boat back to the right and then left, Ellis said. Prop torque only exacerbates the problem.
Inboard boats typically don’t suffer from low-speed wander because their rudders have enough surface area to overcome the tendency to veer off course.
Sterndrives and outboards don’t really have much of a rudder, relying more on directional prop thrust. At low speeds, prop thrust is not as effective as a rudder for maintaining a straight course. And that’s why sterndrives and outboards are more prone to low-speed wander."

Maybe it is just a bit of both here as he does mention "low speed wander" and "Prop torque only exacerbates the problem". It does sound like a form of vortex shedding to me though

[II Lucky]

I added a "super fin" a couple of seasons ago and it virtually eliminated the bow wander. No decrease in performance and much more enjoyable at hull speed.

[seapuppy]

Originally Posted by mcoffey

SP, I am glad to hear you are sticking around. It would have not been the same around here without you.
But on this subject, I will have to agree to disagree with you here. I would like to think that this guy knew what he was talking about (though maybe I put my trust in the wrong people?).
I will quote:
"Roy Ellis, senior project manager of product development for Monterey Boats, said it’s the result of water rushing aft along the sides of the boat and then collapsing into the hole behind it.
This creates swirling vortices of water that exert suction on each side of the transom, pulling the boat back to the right and then left, Ellis said. Prop torque only exacerbates the problem.
Inboard boats typically don’t suffer from low-speed wander because their rudders have enough surface area to overcome the tendency to veer off course.
Sterndrives and outboards don’t really have much of a rudder, relying more on directional prop thrust. At low speeds, prop thrust is not as effective as a rudder for maintaining a straight course. And that’s why sterndrives and outboards are more prone to low-speed wander."

Maybe it is just a bit of both here as he does mention "low speed wander" and "Prop torque only exacerbates the problem". It does sound like a form of vortex shedding to me though

we can always disagree.....the sign of maturity and willingness to see another's point...however being in the airplane business...I see them a bit differently....therefore I disagree with his assessment.......bow wander is simply the rear end trying to push the front end out of theway

but that's just my understanding of what sheer and vorticies do.....
and thanks...I'm glad the wife unit isn't going to make me sell the boat also.....sure would hate to make her hold the anchor one last time

SP